Avoid concurrent package tests (#3073)

See open-telemetry/opentelemetry-collector#3064 (comment) for the reasons for this change. This enforces that package tests don't run concurrently: when they run concurrently it is possible to clash on machine-wide resources like ports.

I tried a few variants to see the effects on CI: CircleCI did get noticeable slow GH Actions too but not as much. While it feels "wasteful" the added time for tests that do not use machine-wide resources, avoiding concurrent run of package tests is a more reliable way to run the tests and avoid test bugs like #3064.

The current choice was the fastest on my machine on runs with a clean test cache.

Fixes #3064

Makefile.Common

@@ -1,7 +1,9 @@
 # ALL_PKGS is the list of all packages where ALL_SRC files reside.
 ALL_PKGS := $(sort $(shell go list ./...))
 
-GOTEST_OPT?= -v -race -timeout 180s
+# Use a single process (-p 1) on go test to avoid tests clashing on machine
+# wide resources, e.g. ports.
+GOTEST_OPT?= -v -p 1 -race -timeout 180s
 GOTEST=go test
 LINT=golangci-lint
 IMPI=impi